Ultra‐High Temperature Mechanical Properties of a Zirconium Diboride–Zirconium Carbide Ceramic

The mechanical properties of a ZrB₂‐10 vol% ZrC ceramic were measured up to 2300°C in an argon atmosphere. Dense billets of ZrB₂‐9.5 vol% ZrC‐0.1 vol% C were produced by hot‐pressing at 1900°C. The ZrB₂ grain size was 4.9 μm and ZrC cluster size was 1.8 μm. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600°C, increasing to 345 MPa at 1800°C and 2000°C, and then decreasing to 290 MPa at 2200°C and 2300°C. Fracture toughness was 4.8 MPa·m^½ at room temperature, decreasing to 3.4 MPa·m^½ at 1400°C, increasing to 4.5 MPa·m^½ at 1800°C, and decreasing to 3.6 MPa·m^½ at 2300°C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200°C, then decreasing linearly to 385 GPa at 1600°C, more slowly to 345 GPa at 2000°C, and then more rapidly to 260 GPa at 2300°C. Surface flaws resulting from machining damage were the critical flaw up to 1400°C. Above 1400°C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000°C, microvoid coalescence ahead of the crack tip caused failure.     

Passive-Oxidation Kinetics of High-Purity Silicon Carbide from 800° to 1100°C

Highly textured chemically vapor-deposited silicon carbide (CVD-SiC) thick films were oxidized and compared to single-crystal SiC and single-crystal silicon. The oxidation rates of the (111) face of the cubic CVD-SiC were the same as those of the (0001) face of the single-crystal SiC. Similarly, the opposite faces of the two materials, (     ) and (000     ), also oxidized at nominally the same rates. The (     ) and (000     ) faces oxidized much faster than their opposite (lll)/(0001) faces. Ellipsometry measurements and kinetic results implied that differences existed between the oxides that grew on the opposite faces. A regression method was developed to analyze the oxide thickness versus time versus temperature behavior of the specimens simultaneously. This technique was compared to typical methods for analyzing temperature-dependent processes and estimated temperature- dependent parameters (e.g., activation energy) and their errors more accurately.     

Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO₂ was used to produce an oxygen partial pressure of ∼10⁻¹⁰ Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO₂ produced a non-protective oxide surface scale.     

Oxidation of Hafnium Carbide in the Temperature Range 1400° to 2060°C

After hafnium carbide has been oxidized at temperatures in the range of 1400° to 2060°C, three distinct layers are present in the film cross section: (a) a residual carbide layer with dissolved oxygen in the lattice, (b) a dense-appearing oxide interlayer containing carbon, and (c) a porous outer layer of hafnium oxide. Experimental measurements of layer thicknesses and oxygen concentrations are combined with an extended formulation of moving-boundary diffusion theory to obtain the diffusion constants of oxygen in each of the three layers. The results indicate that the oxide interlayer is a better diffusion barrier for oxygen than either of the other layers. Based on X-ray microanalysis, X-ray diffraction, and resistance measurements, the interlayer is an oxygen-deficient oxide of hafnium with a carbon impurity. The interlayer hardness equals that of the residual carbide layer.     

Displacement Rate and Temperature Effects in Fracture of a Hot-Pressed Silicon Nitride at 1100° to 1325°C

An MgO-fluxed hot-pressed silicon nitride was fractured in four-point flexure between 1100° and 1325°C at three crosshead speeds. Above 1200°C, a temperature and strain-rate dependence of fracture stress and KIc was seen. Scanning and transmission electron microscopy were used to analyze as received and fractured material. A map of temperature vs crosshead speed was drawn showing regions where subcritical cracking was or was not observed and a transition region where microcracks and voids were detected.     

High-Temperature Thermophysical Properties of Zirconium Carbide

An apparatus based on the method of Jain and Krishnan was developed for property measurements in the temperature range from measurable radiant emission to the thermal stability limit of metallic materials. Apparatus and experimental techniques for cylindrical specimens with length-to-diameter ratios >4 are described. The theory of the method is reviewed, practical formulations are developed, and error sources are analyzed. The following properties were determined for zirconium carbide in the range 1000° to 2500°K: thermal conductivity, spectral thermal emittance at 0.65μ spectral thermal emittance at 0.65μ as a function of time in vacuum, electrical resistivity, and total thermal emittance. A compilation of experimental values for other physical properties of ZrC is given.     

Oxidation Kinetics of Hafnium Carbide in the Temperature Range of 480° to 600°C

The isothermal oxidation of HfC powders was carried out at temperatures of 480° to 600°C at oxygen pressure of 4, 8, and 16 kPa, using an electromicrobalance. The oxidized product was identified by X-ray analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron diffraction, and the morphology of the oxidized grains was observed by scanning electron microscopy. Oxidation proceeds by two processes: a diffusion-controlled process operates up to about 50% oxidation and a phase-boundary-controlled process operates above about 50% oxidation. The activation energies for both processes are the same (197 ± 15 kJ.mol⁻¹). The change in the oxidation process is associated with the generation of cracks on the grains, resulting from the growth or expansion stress due to the formation of monoclinic HfO₂ microcrystallites less than 3 nm in size. In the latter process, the thickness of the diffusion layer is kept constant, being time-independent, which allows the process to apparently obey the phase-boundary-controlled reaction.     

Oxidation Behavior of Chemically-Vapor-Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C

The isothermal oxidation of pure CVD SiC and Si₃N₄ has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha-cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si₃N₄ in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si₃N₄. Silicon oxynitride did not appear to play a role in the oxidation of Si₃N₄ under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.     

Oxidation of Single Crystals of Hafnium Carbide in a Temperature Range of 600° to 900°C

The isothermal oxidation of HfC single crystals with (100) orientation was carried out using an electromicrobalance at temperatures of 600° to 900°C at an oxygen pressure of 2 to 8 kPa. Nonisothermal oxidation was performed by a simultaneous thermogravimetry-differential thermal analysis-mass spectrometry analysis. A polished cross section of the oxidized crystal was observed by backscattered electron imaging in a scanning electron microscope. Quantitative chemical analysis for Hf, O, and C and their elemental profiles in the HfC and oxide scale was carried out by wavelength dispersive X-ray microanalysis. It was found that the oxide scale consists of two regions, zones 1 and 2, both of which showed the existence of carbon. The carbon content at the middle point of zone 1 was about twice that in zone 2, which contained 7 to 14 at.% carbon. Zone 1 showed an almost compact and pore-free phase; its thickness remained constant (1 to 2 μm) after a prolonged time. The thickness of zone 2 increased linearly with time. The oxidation mechanism including interfacial reaction responsible for the deposition of carbon is discussed.     

Time-Dependent Strength of Siliconized Silicon Carbide under Stress at 1000° and 1100°C

The time-dependent strength of a fine-grained siliconized silicon carbide under stress at 1000° and 1100°C was investigated. Both macroscopic stress redistribution and localized flaw blunting were found to contribute to the strengthening of siliconized silicon carbide in bending tests. Strengthening through macroscopic stress redistribution involved nonlinear creep behavior which decreased the maximum outer fiber stress in the bending beam. Localized flaw blunting processes were determined to be operative in this material through artificial flaw tests using a prestress to prevent flaw healing. The sharp artificial cracks were blunted during static load tests by localized deformation processes at the crack tip.     

Corrosion Behavior of Silicon Carbide in 290°C Water

The fundamental corrosion behavior of silicon carbide (SiC) ceramics was investigated after immersion in 290°C water solutions with different pH and dissolved-oxygen concentrations. The weight loss in the oxygenated solution was more than that in the deoxygenated solution and was accelerated by increasing pH. Preferential attack could be found at grain boundaries and around pores on the sample surface immersed in the oxygenated alkaline solution. The weight change, dW, followed the general rate law, (dW)^m= kt. The exponent, m, was 1.11 in the alkaline solution and 0.45 in the acidic solution. Based on the above results, the SiC was considered to be directly hydrolyzed to a silica sol, with its dissolution kinetics dependent on the sol stability. This corrosion behavior is quite different from those in high-temperature or vapor-phase hydrothermal oxidation, where the oxidation rate is controlled by oxidant diffusion through the protective silica surface layer.     

Secondary Phases in Hot-Pressed Aluminum-Boron-Carbon–Silicon Carbide

Silicon carbide hot-pressed with aluminum, boron, and carbon as sintering aids (ABC–SiC), was studied by transmission electron microscopy. Both grain-boundary films and inclusions were prevalent in this material. The present study characterized the inclusions located in triple-junctions, grain boundaries, and the interior of the SiC grains, with emphases on phases not scrutinized before. These inclusions were crystalline, in contrast to the amorphous grain-boundary films. Two dominant types of boron-free triple-junction phases containing Al(Si)-O-C-(S) and Al(Si)-O were identified, where sulfur was an unexpected contaminant, and silicon came from SiO₂ or from dissolution of SiC. Boron-containing inclusions with a composition Al-O-B-C were frequently observed inside SiC grains. Although the boron-free aluminum-rich phases wet the grain boundaries completely and are, therefore, effective sintering additives, the boron-containing Al(Si)-O-B-C did not wet the grain boundaries. The structure and chemical composition of these boron-containing intragranular inclusions were determined, and their mechanism of formation is discussed.     

Optical and Mechanical Properties of Hot-Pressed Cesium Iodide

Polycrystalline CsI disks were fabricated by hot-pressing in a nitrogen-purged glove box. Densification during hotpressing occurred by plastic flow resulting from lattice dislocation glide. Primary recrystallization and extensive grain growth were observed. Both the optical and mechanical properties of this material were significantly affected by grain growth, but in opposite ways. Transmittance increased and strength decreased as grain size increased. The hot-pressed CsI had transmittance of about 85% in the extra-long-wavelength infrared range, a value equivalent to that of single-crystal CsI. The flexural strength of the CsI that was hot-pressed under conditions that minimized grain growth was about 8 times higher than that of single-crystal CsI.     

Synthetic Mica Investigations: IV, Dielectric Properties of Hot-Pressed Synthetic Mica and Other Ceramics at Temperatures up to 400°C.

The dielectric properties of hot-pressed synthetic mica were studied in comparison with those of other ceramic insulating materials with special attention to the variation of these properties with temperatures up to 400° C. Power-factor and dielectric-constant measurements were made at 1 mc., and the volume resistivity was determined on a d.-c. resistance bridge with a voltage of 22.5. The comparison revealed the importance of evaluating the electrical properties of a dielectric material at the temperature at which the insulator is expected to operate. A plot of the power-factor data against the reciprocal of the absolute temperature did not permit the results to be represonted by two intersecting straight lines, as has been previously reported.     

Corrosion of CVD Silicon Carbide in 500°C Supercritical Water

A high-purity CVD β-SiC showed a relatively low corrosion rate in deoxygenated supercritical water at 500°C. The corrosion rate was lower than that previously reported for CVD SiC in 360°C water and much lower than that reported for sintered and reaction-bonded SiC. The present study confirmed that CVD SiC was preferentially attacked at the grain boundaries. Analytical examinations did not reveal the presence of a measurable oxide scale. As a result, it is believed that corrosion of the high-purity SiC occurred via hydrolysis to hydrated silica species at the surface that were rapidly dissolved into the supercritical water.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

The System Magnesia-Silica-Water Below 300°C.: I, Low-Temperature Phases from 100° to 300°C. and Their Properties

Reactions in the ternary system MgO-SiO₂-H₂O were studied over the temperature range 100° to 300° C. and were found to produce only two phases. Under conditions of 100° to 200° C. and atmospheric pressure up to 20,000 lb. per sq. in., and regardless of the initial MgO/SiO₂ ratio, the predominant magnesium silicate product was found to have a MgO/SiO₂ ratio of 1.5. In the range 200° to 300°C. at 210 to 20,000 lb. per sq. in. two stable phases, 3MgO.2SiO₂.2H₂O (I) and 3MgO.4SiO₂.H₂O (II), were observed. In this case the phase that was favored was determined by the molar ratio of MgO/SiO₂ of the reaction mixture at the start of the run. The physical and chemical properties of phases (I) and (II) resembled those of the natural minerals serpentine and talc respectively. Two different morphologies were observed in electron micrographs of phase (I). From 100° to 160°C. it occurred as crumpled foils, and at 170°C. fibrous crystallites, which resembled the natural asbestos mineral chrysotile, appeared at the expense of some of the foils.     

Microstructure of Chromite-Periclase at 1650° to 2310°C

Progressive solid solution of chromite in periclase occurs with increasing temperature. Solid solution is virtually complete except for a few coarse remnants at 1980° C and these are gone at 2090° C. Specimens cooled slowly exhibit exsolution structures, whereas specimens quenched from the higher temperatures consist essentially of a single solid-solution phase. Two periclase structure phases are indicated by X-ray patterns in some quenched specimens. Silicate alters the relations between chromite and periclase in specimens cooled slowly and facilitates segregation of spinel from the solid solution in quenched specimens. Textural features observed in electrically fused specimens are similar to those resulting from predominantly solid-state reactions in specimens fired to 2200°C.     

Microstructure and Mechanical Properties of Hot-Pressed Silicon Carbide-Aluminum Nitride Compositions

A flexural strength of up to 1 GPa was achieved in SiC-AIN materials and is attributed to a dense, equiaxial grain structure of the 2H(δ) SiC-AIN solid solution, with a relatively uniform grain size of ∼ 1 μm. The strength was found to decrease with increasing grain size. While the β→α phase transformation and the formation of various metastable polytypes make microstructural control difficult in SiC materials, excellent control is facilitated in SiC-AIN materials as a result of the stable 2H solid solution. Several mechanisms of grain refinement during the β→ 2H transition were observed, most notably the direct formation of several 2H grains from a single β grain. In addition, grain growth is limited by the diffusion-controlled nature of the transition. These mechanisms could be utilized to achieve even higher strength values, with potentially higher reliability of the materials in structural applications.